Super-Eddington Fluxes from Thin Accretion Disks?

Abstract

Radiation pressure-dominated accretion disks are predicted to exhibit strong density inhomogeneities on scales much smaller than the disk scale height, due to the nonlinear development of photon bubble instability. Radiation would escape from such a "leaky" disk at a rate higher than that predicted by standard accretion disk theory. The disk scale height is then smaller than that of a similar disk without small-scale inhomogeneities, and the disk can remain geometrically thin even as the flux approaches and exceeds the Eddington limit. An idealized one-zone model for disks with radiation-driven inhomogeneities suggests that the escaping flux could exceed L_E by a factor of up to ~10-100, depending on the mass of the central object. Such luminous disks would develop strong mass loss, but the resulting decrease in accretion rate would not necessarily prevent the luminosity from exceeding L_E. We suggest that the observed "ultraluminous X-ray sources" are actually thin, super-Eddington accretion disks orbiting stellar-mass black holes, and need not indicate the existence of a class of intermediate-mass black holes.